1. Field of the Invention
This invention relates to a method of fabricating a solar cell, and more particularly to a method of fabricating a back surface point contact silicon solar cell.
2. Description of the Related Art
The silicon solar cell generates electrical charge when exposed to solar radiation. The radiation interacts with atoms of the silicon and forms electrons and holes which migrate to p-doped and n-doped regions in the silicon body and create voltage differentials between the doped regions. U.S. Pat. No. 4,234,352 discloses a solar energy convertor which includes a parabolic cone radiation concentrator portion and a processor portion including a radiator that absorbs concentrated radiation and generates incandescent radiation. A silicon solar cell receives the incandescent radiation and generates the voltage differentials between the doped regions. U.S. Pat. No. 4,927,770 discloses a back surface point contact silicon solar cell to be suitable for the concentrator solar cell.
Since the concentrator solar cells generate a lot of current (e.g., 10 A/cm.sup.2 or more for concentration=200 to 500) and have a low voltage (e.g., 0.8 V), the series resistance of the solar cell must be small (such as less than 0.003 .OMEGA.-cm). To attain this very low value of series resistance, the metallization of the solar cell should have a double layer of metallization as described in the aforesaid patent (No. 4,927,770).
In this kind of solar cell, the first layer of metallization contacts the semiconductor positive and negative contacts (the p-doped and n-doped regions) in a very fine pattern to insure a high efficiency under high concentration. The second layer of metallization maintains a low series resistance and must be solderable. In between these two layers of metallization, there must be a layer of an insulator (dielectric) material such as silicon oxide or alumina oxide as is disclosed in the aforesaid patent (No. 4,927,770).
In the concentrator silicon solar cell, because of the high concentration ratio (e.g., .times.200 to 500, or incident power density of 20 to 50 W/cm.sup.2), the first metal layer is very thick (e.g., 2 to 4 .mu.m). This high thickness of the first metal layer and the intermetal insulator may sometimes make proper deposition of the second thin metal layer (e.g., 1 to 2 .mu.m) over the patterned insulator layer difficult, causing the second metal layer to have poor conductivity or worse, to break.
Moreover, when soldering the cell onto a metallized substrate (made of aluminiumnitride (ALN) or alumina (Al.sub.2 O.sub.3), for example), the formation of voids (i.e., bubbles, pinholes or cracks) is much greater on a non-smooth cell surface. In other words, less voids will happen during soldering when the cell surface is well planarized. Also, the solder fatigue due to the difference in thermal expansion is much less on the smooth surface than on a surface with severe topography (unevenness). Thus, the surface of the cell to be soldered onto a metallized substrate should preferably be as planarized and even as possible.